This invention relates to load control systems such as bridge circuits, and in particular to improvements which provide failsafe protection.
One type of commonly used load control system includes a bridge circuit which can be controlled to apply a voltage of either polarity to a load such as a motor. U.S. Pat. Nos. Iordanidis, 3,525,883, Korte, 4,066,945, Levine, 4,063,141, Cathell, 4,105,957, Black, 4,298,926, Kade, 4,545,004, and Damiano, 4,616,305 provide examples of several prior art circuits.
Load control systems incorporating such bridge circuits are subject to two potential failure modes. First, a failure in the system controller can inadvertently cause the load to be powered for an excessively long time period. For example, a load control system which uses a controller to set latches in a remote station to control a load can suffer a controller failure which leaves the latches set. Watchdog timers have been used in the past to remove voltage from the load if the controller fails to generate a periodic failsafe signal. One object of this invention is to provide a failsafe system which can be used in conjunction with a bridge circuit comprising four n-channel MOSFETs without significantly increasing the cost or complexity of the overall system.
Another failure mode of bridge circuits is that improper operation of the switches (due, for example to electromagnetic interference or controller failure) can create a direct short through the bridge circuit. Such a short, if it lasts for any appreciable time, can of course destroy the bridge circuit. The Iordanidis and Korte patents mentioned above recognize this problem, and both disclose feedback systems that are responsive to the voltage drop across the load or the current passing through the bridge to prevent three or more of the switches from being closed simultaneously. This approach is responsive to the controlled current rather than to the load control signals, and for this reason is only indirectly responsive to the load control signals themselves. It is an object of this invention to provide a circuit which simultaneously applies each of the load control signals to close a pair of diagonally opposed switches and to disable the complementary pair of switches, thereby preventing short circuits in the bridge.